Immunostaining of interfollicular large B cells for memory and plasma cell markers. (A) Staining for memory markers. The memory cell marker CD27 analyzed on interfollicular large B cells in the tonsil by combining immunoenzymatic (OCT-2 blue) and double immunofluorescence staining (CD27 green; CD20 red). (i) Low-power view (paraffin section; magnification, × 20) shows that interfollicular large B cells are OCT-2⁺ and CD20⁺, but are CD27^– (arrows). In contrast, T cells (green) are strongly CD27⁺. (ii) Two OCT-2⁺ and CD20⁺ interfollicular large B cells in the same section are shown at higher magnification (× 40): one of them (arrow) appears to be weakly CD27⁺. (iii) Two OCT-2⁺ and CD20⁺ (arrows) interfollicular large B cells in a frozen tissue section are clearly CD27^–. Magnification, × 40. (B) The plasma cell–associated markers p63 (antibody VS38c) (as also shown at higher magnification in the insets) and CD38 are not expressed by interfollicular large B cells. Left panel: double immunofluorescence staining (CD20 red; VS38c green). Magnification, × 20. Right panel: double immunoenzymatic staining (CD20 blue; CD38 brown). Magnification, × 10. (C) In contrast, MUM-1 (IRF-4), a transcription factor found in many plasma cells and in scattered cells in the light zone of germinal centers, is seen in some interfollicular large B cells (left panel; magnification, × 20). Double immunoenzymatic staining (left panel, inset) shows 3 out of 4 cells coexpressing CD20 (blue) and MUM-1 (brown). Magnification, × 40. On the right, double immunofluorescence labeling (CD20 red; MUM-1 green) of an interfollicular area confirms that a proportion of CD20⁺ cells coexpress MUM-1 (as also shown at higher magnification in the insets). Note the markedly dendritic profile of the arrowed cell. Magnification is × 40 for all insets.